1. Field of the Invention
The present invention relates generally to a protection method and apparatus for thyristors in a power converter and more particularly to a protection method for thyristors using forced triggering signals.
2. Discussion of the Background
Thyristors are commonly used in power converters and similar applications. The thyristors may be arranged in a series connection or a series-parallel connection. While these devices have become common, a certain type of failure may occur when an unusual transient pulse occurs during a certain period of operation of the device.
FIG. 1 shows a prior art thyristor converter that is used for direct current power transmission. Incoming power is converted from alternating current to direct current or vice versa utilizing transformer 12 and thyristor converter 10. Reactor 13 helps to smooth the resultant current. An arm of the thyristor converter 10 is shown in FIG. 2 as including a number of thyristor converters 16.sub.1..16.sub.I..16.sub.N arranged in series. The thyristors shown are light thyristors that are also connected to individual voltage dividing circuits 18.sub.1..18.sub.I..18.sub.n which serve to make the thyristor voltages uniform. Light guides 14.sub.1..14.sub.I..14.sub.N are light guides which transmit the trigger light signals from a pulse generator (not shown) to the respective thyristors. Arrestor 15 serves to suppress thyristor overvoltage. The number of thyristors, N, which are arranged in series is determined by the voltage rating of the converter.
FIG. 3 shows graphs of thyristor voltage and current over a period of time, when operating as an inverter. Circuit voltage E.sub.t, determined by the main circuit, is imposed on the thyristor. At time t.sub.1 this voltage falls to zero and the thyristor becomes conductive as seen by the graph of the thyristor current. At time t.sub.2 the thyristor is turned off and circuit voltage E.sub.t is made negative. Since carriers remain inside the thyristor immediately after conduction, it is not possible to immediately achieve a forwardly-directed withstand voltage (forward recovery), but it is necessary first to impose a set reverse voltage period until the carriers are removed. This reverse voltage period is known as the margin angle, .gamma.. If this reverse voltage period is too small, commutation failure may occur in the converter as a whole or there may be an occurrence of partial commutation failure known as partial self-triggering. This occurs when series connected thyristors include some elements in which triggering occurs and some elements where it does not occur. When this happens, the entire circuit voltage is then imposed on the elements where triggering does not take place. This results in the element breaking down due to overvoltage or element breakdown due to a self-triggering domino effect which accompanies a rise in voltage above a certain level. In order to compensate for this, other prior art devices have made use of forced triggering protection where triggering signals are forcibly supplied if the margin angle becomes too small due to a decrease in system voltage, voltage distortion or improper control. This protection helps prevent excessive stress on elements and prevents element breakdown. Thus, forced triggering is known to prevent self-triggering of thyristors during times of overvoltage or when the voltage is above a dangerous level. However, this approach has only been used during the margin angle (reverse voltage) period. Specifically, it has not proved satisfactory when overvoltage enters the converter immediately after conduction.
At time t.sub.2 shown in FIG. 3, E.sub.t assumes a reverse voltage to start the margin angle. This period ends at time t.sub.3, when E.sub.t becomes zero.
The thyristor recovery voltage E.sub.tf gradually increases when the residual carriers disappear during the margin angle. This voltage starts to increase from zero at a time t.sub.20. This voltage increases and eventually crosses the voltage level V.sub.pt which corresponds to the protection level provided by the arrestor 15. It then continues until it reaches the forward withstand voltage V.sub.DRM at time t.sub.6. For a high withstand voltage, large current thyristor, the time necessary to achieve forward recovery varies considerably within the range of 1.2 to 1.5 T.sub.q, where T.sub.q is the on-off time. This forward recovery time is seen in the FIGURE as the time between t.sub.2 and t.sub.6.
As can be seen from FIG. 3, in normal operation E.sub.tf is greater than E.sub.t during the forward recovery time. Thus, the thyristor forward recovery voltage is always higher than the circuit voltage. However, during this time if a forwardly directed overvoltage occurs such as indicated by the dashed line at t.sub.40, the circuit voltage may become greater than the forward recovery voltage for a short time. The thyristor may be unable to withstand this lower voltage and accordingly, the problem of self-triggering occurs. The thyristor will then breakdown if the voltage is above a certain value (designated as the limit self-triggering voltage V.sub.stl). V.sub.stl may be approximately one-third to one-half of V.sub.DRM. Since the occurrence of an overvoltage is random and may occur at any time, it is necessary to protect the thyristor elements during the time period of t.sub.2 to t.sub.5. After t.sub.5 there is no problem since there is satisfactory recovery to the forward withstand voltage. During the time period t.sub.2 to t.sub.3 (the margin angle), it is possible to utilize conventional force triggering protection as described above. However, during the time period t.sub.3 to t.sub.5, prior art devices cannot provide protection. Accordingly, during this vulnerable period the thyristors may breakdown due to overvoltage.